Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as described herein can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a mask pattern is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g., an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection systems, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441, incorporated herein by reference.
The photolithographic masks referred to above comprise geometric patterns corresponding to the circuit components to be integrated onto a silicon wafer. The patterns used to create such masks are generated utilizing CAD (computer-aided design) programs, this process often being referred to as EDA (electronic design automation). Most CAD programs follow a set of predetermined design rules in order to create functional masks. These rules are set by processing and design limitations. For example, design rules define the space tolerance between circuit devices (such as gates, capacitors, etc.) or interconnect lines, so as to ensure that the circuit devices or lines do not interact with one another in an undesirable way. The design rule limitations could be referred to as “critical dimensions” (CD). A critical dimension of a circuit can be defined as the smallest width of a line or hole or the smallest space between two lines or two holes. Thus, the CD determines the overall size and density of the designed circuit. Of course, one of the goals in integrated circuit fabrication is to faithfully reproduce the original circuit design on the wafer (via the mask).
An important aspect to be considered in the overall photolithography process is the ability of the mask writing apparatus (referred to herein as a mask-writer) to accurately produce the target mask design. In other words, the ability to transform the target mask design into a mask that can be utilized in the actual imaging process. Known mask writer devices or units include e-beam mask writers and optical mask writers, each of which have different underlying imaging physics and different variable parameters which affect the results of the mask writing process. For example, e-beam mask writers have parameters such as, but not limited to, beam size, focus, beam dose, beam current, beam energy, acceleration voltage, and beam blur and, parameters to correct for proximity effects caused by, for example, back-scattered electrons, fogging affects and pattern dependent processing steps such as etching. Optical mask writers have parameters, such as, but not limited to, numerical aperture (NA), focus, illumination shape and dose. In each instance, some of the parameters of the given mask-writer are fixed and some are tunable. The tunable parameters of the given mask writer unit can be adjusted in an effort to improve the results of the mask writing process.
Another goal is to be able to utilize different mask-writer units to produce the same target mask for imaging a desired pattern without having to expend considerable amounts of time and resources determining the necessary settings of each mask writer unit to achieve optimal/acceptable performance. As is known, designers/engineers spend a considerable amount of time and money determining the optimal settings of a given mask writer unit when initially setting up the given mask writer unit so that the resulting mask satisfies the design requirements. Indeed, this is often a trial and error process wherein the tunable parameters on the mask writer unit are selected and the mask generated and then analyzed to determine if the resulting mask is within specified error tolerances. If not, the tunable parameters are adjusted and the mask is generated and analyzed again. This process is repeated until the resulting mask is within the specified error tolerances.
However, as each mask writer unit, even identical model types, exhibit for example different proximity effects when generating a mask, the actual mask which is generated often differs from mask writer unit to mask writer unit. For example, different optical proximity effects (OPEs) associated with given optical mask writer units can introduce significant CD variations through pitch. As such, it is not possible to simply utilize any mask writer unit to generate a given mask, as the resulting mask can vary considerable. Thus, if it is desirable to utilize a different mask writer unit to form a given mask, the engineers must optimize or tune the new mask writer unit, so that the resulting mask formed by the mask writer unit satisfies the design requirements. Currently, this is typically accomplished by a trial and error process, which as noted above, is both expensive and time consuming.
As such, there is a need for a method for tuning or optimizing a given mask writer unit that allows the mask writer unit to produce a mask within a specified error tolerance relative to a previously tuned mask writer unit such that both mask writers are effectively capable of producing the same mask. In other words, there is a need for a method for optimizing the performance of multiple mask writers with respect to a given target mask that does not require a trial and error optimization process and which allows all mask writer units to produce masks within a predefined error tolerance.